1. Field of the Invention
The present invention relates to a solid state image sensor and a method of driving the same.
2. Description of the Related Art
FIG. 1 is a diagram illustrating the structure of an interline transfer type solid state image sensor. In the interline transfer type solid state image sensor shown in FIG. 1, on a semiconductor substrate 801 are arranged groups of vertical charge transfer registers 802, groups of photoelectric converting elements 803, and a group of horizontal charge transfer registers 804. Each of the groups of vertical charge transfer registers 802 is composed of a plurality of charge transfer registers in a vertical direction. Each of the group of photoelectric converting elements 803 is provided for one of the groups of vertical charge transfer registers 802. The group of horizontal charge transfer registers 804 are electrically connected to the one end of each of the groups of vertical charge transfer registers 802. Also, a charge detecting section 805 is provided to be electrically connected to the one end of the group of horizontal charge transfer registers 804. A signal outputted from the charge detecting section 805 is outputted outside through an output terminal 806.
FIG. 2 is a diagram to explain the arrangement of transfer electrodes of the group of vertical charge transfer registers and the photoelectric converting elements and the connection between bus lines and the transfer electrodes. The solid state image sensor shown in FIG. 2, the group of vertical charge transfer registers are driven in 4-phase pulses signal. In FIG. 2, four vertical transfer electrodes 901 are provided for one photoelectric converting element 803 to form a unit pixel. The vertical transfer electrodes 901 are connected to the same bus line 902 for every four electrodes. Also, a horizontal transfer electrode 903 is provided for the one end of the group of vertical charge transfer registers.
In such a solid state image sensor, two operations could be needed, one is the operation in which the signal charge of each photoelectric converting element is individually read, and the other is the operation in which the signal charges of two adjacent photoelectric converting elements in the vertical direction are added and read. When the signal charges of two photoelectric converting elements are added and outputted, the resolution of the solid state image sensor decreases. However, the time required to output the signal charges of all the photoelectric converting elements can be made short. Also, when the solid state image sensor is driven in an interlace mode, the signal charges of two adjacent photoelectric converting elements in the vertical direction are added and outputted.
In the solid state image sensor, as a method of switching between an individual pixel read mode and a 2-pixel read mode in which the signal charges of two adjacent photoelectric converting elements in the vertical direction are added and read, there is a method in which the signal charges in the groups of vertical charge transfer registers are transferred twice in a horizontal blanking period. This method is described in Japanese Laid Open Patent Application (JP-A-Heisei 4-262679). The charge transfer of the group of vertical charge transfer registers in the horizontal blanking period is performed once in the individual pixel read mode (the signal charge for one pixel is transferred) and twice in the 2-pixel read mode (the signal charges for two pixels are transferred).
FIGS. 3A to 3E are timing charts of the pulses which are applied to the vertical transfer electrodes and the horizontal transfer electrode during the horizontal blanking period in the individual pixel read mode. In this case, only the pulse .phi.H1 which is applied to the horizontal transfer electrode 903 is shown as a horizontal drive pulse. FIGS. 4A to 4I are diagrams illustrating the accumulation states of the signal charges in the group of vertical charge transfer registers at each timing of FIGS. 3A to 3E.
When a high voltage pulse is applied once to the vertical transfer electrodes during the horizontal blanking period, the signal charge is read from the photoelectric converting element 803 to the vertical charge transfer register 802 and then is transferred for one pixel in the group of vertical charge transfer registers. At this time, the signal charge of the last stage of vertical charge transfer register is transferred to the horizontal charge transfer register. When receiving the signal charges for one row from the groups of vertical charge transfer registers, the group of horizontal charge transfer registers transfer the received signal charges to the output section in order. By repeating the above operation for all pixels in the vertical direction, the signal charges of all the pixels of the imaging region are outputted.
FIGS. 5A to 5E are timing charts of the pulses which are applied to the vertical transfer electrodes and the horizontal transfer electrode during the horizontal blanking period in the 2-pixel read mode. Also, FIGS. 6A to 6M are diagrams illustrating the accumulation states of signal charges in the groups of vertical charge transfer registers at each timing of FIGS. 5A to 5E.
When a pulse is applied twice to each vertical transfer electrode during the horizontal blanking period, the signal charges are read from the photoelectric converting element 803 to the group of vertical charge transfer registers 802 and then are transferred for 2 pixels in the group of vertical charge transfer registers. At this time, the signal charge of the last stage of vertical charge transfer register and the signal charge of and the stage previous to the last stage of vertical charge transfer register are continuously transferred to the group of horizontal charge transfer registers to be added to each other in the horizontal charge transfer register. When receiving the signal charges for 2 pixels from the group of vertical charge transfer register, the group of horizontal charge transfer registers transfer the received signal charges to the output section in order. By repeating the above operation for a half of the number of pixels in the vertical direction, the signal charges of all the pixels of the imaging region are outputted.
By the way, when a pulse is applied twice to each vertical transfer electrode during the horizontal blanking period in the 2-pixel read mode, the width of the pulse becomes short. In the conventional example of the solid state image sensor, when the width of the pulse becomes short, the amplitude of the drive pulse becomes substantially small because of the influence of the pulse propagation delay by the capacity and resistance of the transfer electrode. For this reason, the maximum transfer quantity of signal charge by the vertical charge transfer register decreases.
Also, if the horizontal blanking period is made long so as to suppress the pulse propagation delay, the time given to output the signal charges from the group of horizontal charge transfer registers becomes short. Therefore, it is necessary to increase the frequency of pulses used to drive the group of horizontal charge transfer registers. As a result, the transfer failure in the group of horizontal charge transfer registers, and the increase of the power consumption in a driving circuit of the solid state image sensor.
In addition to the above reference, a driving method in an solid state image sensor is known in Japanese Laid Open Patent Application (JP-A-Heisei 2-196567). In this reference, a plurality of photoelectric converting elements are arranged in a matrix manner to form pixels. A vertical transfer stage is provided for each of the plurality of photoelectric converting elements and reads a signal charge from the photoelectric converting element and transfers in the vertical direction. A horizontal transfer stage is supplied with the signal charges transferred from a vertical transfer means composed of a plurality of vertical transfer stages in the vertical direction. The signal charge is transferred for two vertical transfer stages during one horizontal blanking period such that the signal charges are added in the horizontal transfer stage.